1. Field of the Invention
The present invention relates to a process for the preparation of acetic anhydride by the carbonylation of methyl acetate.
2. Description of the Prior Art
It is well known that acetic anhydride can be produced by the carbonylation of methyl acetate under relatively severe pressure conditions, in the presence of nickel complexes of the formula EQU [A.sub.4 M].sub.2 NiX.sub.4
in which X represents a bromine or iodine atom, M represents a phosphorus or nitrogen atom and A is, for example, a lower alkyl radical; compare U.S. Pat. No. 2,729,651. These complexes, which are obtained by reacting nickel halides with quaternary phosphonium or ammonium halides, can be used in this form in the reaction in question, or alternatively they can be formed in situ. However, the efficiency of this type of process is low, despite the high pressures used.
More recent work (compare British Patent Application No. 2,007,666) has shown that substantially milder conditions can be used during the carbonylation of methyl acetate in the presence of nickel, iodine (or an iodine compound) and a promoter chosen from among organic compounds of the trivalent elements of the nitrogen group; considerable amounts of acetic anhydride can then be obtained provided that the following two conditions are satisfied simultaneously:
the iodine (or the iodine compound) must be used in a proportion such that the fraction of iodine which is not chemically bonded either to the nickel or to the promoter is at least 0.2 mol (of elementary iodine) per mol of both the nickel compound and the promoter; and
the reaction in question must be carried out in an aliphatic carboxylic acid as the solvent.
From an examination of the techniques proposed earlier, it is apparent that it would be desirable to be able to produce acetic anhydride efficiently by the carbonylation of methyl acetate, in the presence of a nickel-based catalyst system, without using organic solvents foreign to the reaction medium, while at the same time carrying out the reaction under relatively mild pressure conditions. It would also be very desirable to improve the productivity of the nickel-based catalyst system.